Protein Degradation

Synthesis is not the only determinant of the complement of proteins present at any time within a cell. Proteins have limited life times and may be degraded within a few minutes to a few years after synthesis. Each cellular protein has a characteristic half-life, but changing physiological circumstances may shorten or lengthen life spans of some proteins; protein degradation is a regulated process. Most cellular proteins are destroyed by the ubiquitin-proteosame proteolytic pathway. Proteins destined for destruction are tagged by the covalent addition of multiple copies of a 76-amino-acid peptide called ubiquitin, which is named for its ubiquitous presence in all eukaryotic cells. Understanding of the factors that guide selection of any particular protein molecule for destruction is incomplete. Ubiquinated proteins attach to and are unfolded and threaded into a barrel-shaped complex of proteins called the protea-some. Proteolytic activity on the inner surface of the proteasome cleaves proteins into small fragments that are released into the cytosol along with intact ubiquitin, which can then be recycled. The fragments are taken up by the lysosomes and degraded to free amino acids. The overall process requires an input of energy derived from the breakdown of ATP and involves a series of enzymatic transformations. Lysosomes also degrade misfolded or otherwise impaired proteins immediately after synthesis. Extracellular proteins, including signal molecules and some membrane proteins, are taken up by endocytosis and transferred to lysosomes, where they are degraded.

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